1. Field of the Invention
This invention relates to the field of photoelectrochemistry and, in particular, to photoelectrochemical cells having coated semiconductor electrodes which enhance the efficiency of the photoelectrochemical devices. More specifically, the present invention relates to protective and catalytic coatings for semiconductor electrodes including particulate semiconductor microelectrode systems.
2. Description of the Prior Art
The field of photoelectrochemistry is recognized as having the potential to enable solar energy utilization to meet many of the energy needs of the future. Through the action of light, photoelectrochemical cells can be used to generate electric power and/or to synthesize fuels and desired chemicals from abundant, renewable resources such as water, nitrogen, and carbon dioxide.
Photoelectrochemical cells can be configured such that one or both electrodes are photoactive semiconductors. The electrodes are in contact with an electrolyte which may be in liquid form or may also comprise a solid polymer matrix. A junction is formed at the semiconductor-electrolyte interface in the dark as the two phases come into electronic equilibrium such that the Fermi level of the semiconductor, E.sub.f, equals the electrochemical potential of the solution, E.sub.redox, producing a barrier height which depends on the nature of the solution species and the specific semiconductor. Upon illumination of the semiconductor with light energy equal to or greater than that of the semiconductor bandgap, electrons are promoted from the valence band to the conduction band, creating electron-hole pairs at or near the interface. The electron-hole pairs are spatially separated by the semiconductor junction barrier, and are injected into the electrolyte at the respective electrodes to produce electrochemical oxidation and reduction reactions.
When electrical power is the only output of the cell, only one effective redox couple is present in the electrolyte. The reaction occurring at one electrode is the reverse of that at the other and no net chemical change occurs in the electrolyte (.DELTA.G=O). Photoelectrochemical cells for solar photolysis and photoelectrolysis contain at least two different redox couples, and light energy induces a current flow that produces a net chemical change in the electrolyte (.DELTA.G.noteq.O). If .DELTA.G of the net electrolyte reaction is negative, the process is exergonic, and light energy only provides the activation energy for the thermodynamically downhill reaction. If .DELTA.G of the net reaction is positive, the process is endergonic, and light energy is converted into chemical energy. When the valence-band holes and the conduction-band electrons have sufficient energy for the respective oxidation and reduction half-reaction, no supplementary external voltage is required and the cell operates spontaneously. An example of such a known process is the photolysis of water into H.sub.2 and O.sub.2 with suspended platinized TiO.sub.2 particles. The semiconductor particles behave as a short-circuited photocell. The illuminated TiO.sub.2 surface acts as the photoanode to oxidize water to O.sub.2, and the Pt region serves as an effective reduction site for hydrogen formation. Several relatively stable semiconductors such as SrTiO.sub.2, KTaO.sub.3, and Nb.sub.2 O.sub.5 require no external bias to generate H.sub.2 and O.sub.2. However, such semiconducting oxides have large bandgaps (3.4-3.5 eV; 365 nm-354 nm) and absorb very little of the terrestrial solar spectrum. U.S. Pat. Nos. 4,090,933 and 4,011,149 are exemplary of prior art cells for the photoelectrolysis of water using solar energy and teach the use of an external bias of from 0 V to about 1 V. An economically viable photoelectrochemical solar cell will probably require solar conversion efficiencies above 10% and long-term stability. Efficient conversion of solar light energy to electrical power requires the optimization of the product of the external photovoltage and photocurrent.
A theoretical maximum of about 30% efficiency is reached for a bandgap of 1.3 eV (.lambda.=960 nm) and exceeds 20% in the 1.7 to 1.1 eV (.lambda.=735 to 1380 nm) range. Theoretical efficiencies higher than 50% have been determined for photoelectrochemical cells consisting of multiple semiconductors, each absorbing part of the solar spectrum. The efficiency for the generation of fuels and desired chemicals will depend on the specific process. For example, thermodynamically, the electrolysis of water at standard conditions requires 1.23 eV per charge, and depending on the current density of the cell, a minimum of about 0.3 to 0.4 eV additional energy is required to sustain the reaction. Thus, the minimum average bandgap of semiconductors must be at least 1.5 eV. The maximum bandgap to achieve solar conversion efficiencies above 10% is likely to be less than 3.0 eV, preferably less than 2.3 eV for a single-photoelectrode-based cell.
A major impediment to the exploitation of photoelectrochemical cells in solar energy conversion and storage is the susceptibility of small bandgap semiconductor materials to photoanodic and photocathodic degradation. The photoinstability is particularly severe for n-type semiconductors where the photogenerated holes, which reach the interface, can oxidize the semiconductor itself. In fact, all known semiconducting materials are predicted to exhibit thermodynamic instability toward anodic photodegradation. Whether or not an electrode is photostable then depends on the competitive rates of the thermodynamically possible reactions, namely, the semiconductor decomposition reaction and the electrolyte reactions.
Examples of photoanodic decomposition reactions are compiled in Table I.
TABLE 1 ______________________________________ Examples of Photoanodic Decomposition Reactions of Various Semiconductor Electrodes Semi- con- Decomposition ductor Photoanodic Process ______________________________________ Si Si + 4h.sup.+ + 2H.sub.2 O.fwdarw.SiO.sub.2 + 4H.sup.+ GaAs GaAs + 6h.sup.+ + 5H.sub.2 O.fwdarw.Ga(OH).sub.3 + HAsO.sub.2 + 6H.sup.+ GaP GaP + 6h.sup.+ 6H.sub.2 O.fwdarw.Ga(OH).sub.3 + H.sub.3 PO.sub.3 + 6H.sup.+ CdS CdS + 2h.sup.+ .fwdarw.Cd.sup.2+ + S CdSe CdSe + 2h.sup.+ .fwdarw.Cd.sup.2+ + Se MoS.sub.2 MoS.sub.2 + 18h.sup.+ + 12H.sub.2 O.fwdarw.MoO.sub.3.sup.2- + 2SO.sub.4.sup.2- + 24H.sup.+ WO.sub.3 WO.sub.3 + 2h.sup.+ + 2H.sub.2 O.fwdarw.WO.sub.4.sup.2- + 1/2O.sub.2 + 4H.sup.+ ______________________________________
Photoanodic instability of the semiconductor entails ionic dissolution, gas evolution, and/or formation of a new phase of the electrode that may block charge transmission to the electrolyte. Both solvation effects and multiple-hole reactions are involved in the decomposition mechanism. Several approaches have been employed to suppress the photocorrosion of n-type photoanodes utilized for the generation of electrical power. By suitable selection of a redox couple, the photogenerated holes can be removed rapidly before corrosion occurs. For example, the addition of S.sub.n.sup.2- to the electrolyte suppresses the photocorrosion of CdS. Among the factors implicated in the stabilizing action are the more favorable redox potential for hole transfer to S.sub.n.sup.2- compared with self oxidation of CdS; preferential adsorption of S.sub.n.sup.2- on CdS and the concomitant shielding of the surface atoms from the solvent; the common ion effect; the favorable kinetic behavior of the chalcogenide redox couple, which facilitates hole abstraction; and the surface morphology. The addition of one or more polychalcogenide ions (S.sub.n.sup.2-, Se.sub.n.sup.2-, or Te.sup.2-) has been used to stabilize CdS, CdSe, CdTe, GaAs, and InP. A variety of other reducing agents [I.sub.3.sup.-, Fe(CN).sub.6.sup.4-, Fe.sup.2+, Ce.sup.3+, etc.] have also been employed to scavenge the photogenerated holes at rates that suppress anodic decomposition.
It is also known that photodecomposition can be suppressed by using nonaqueous solvents, molten salts, and high concentrations of electrolytes. In part, these methods are intended to reduce the solvation effects of water and thus to shift the photodecomposition potential to positive values. Other approaches to stabilize the semiconductor have relied on a high concentration of a redox couple in the electrolyte, specific adsorption of a species acting as a charge relay, or covalent attachment of a charge mediator to the electrode surface. These methods are designed to facilitate charge removal and thus to reduce the steady-state population of photogenerated holes at the interface.
The range of approaches for suppression of the photocorrosion problem in cells for chemical production is more severe than that for electricity-generating cells. This is particularly true if the electrolyte contains water. Table II illustrates some examples of fuel producing reactions in aqueous electrolytes.
TABLE 2 ______________________________________ Some endergonic fuel generation reactions starting with N.sub.2, CO.sub.2, and H.sub.2 O H.degree. (kJ G.degree. Reaction mol.sup.-1).sup.a (kJ mol.sup.-1).sup.a ______________________________________ ##STR1## 286 237 ##STR2## 270 286 ##STR3## ##STR4## 563 522 O.sub.2 (g) ##STR5## 727 703 ##STR6## ##STR7## 890 818 2O.sub.2 (g) ##STR8## 765 678 ##STR9## ##STR10## 467 480 ##STR11## ______________________________________ 1 eV = 23.06 K cal/mol = 96.485 kJ/mol 1 J. = 0.23901 cal
Water is a particularly attractive source of hydrogen for the reduction of materials such as N.sub.2 and CO.sub.2 as well as for the direct generation of H.sub.2. Water can only be used, however, if the semiconductor electrodes are stable in its presence. In the illustrations, the production of energy-rich materials (e.g., H.sub.2, CH.sub.3 OH, CH.sub.2 O, CH.sub.2 O.sub.2, and NH.sub.3) is associated with O.sub.2 evolution. A major problem in photoelectrochemistry is that the oxidation of water at the photoanode of nonoxide n-type materials is thermodynamically and kinetically disfavored over the reaction of the valence band holes with the semiconductor lattice. In fact, all known monoxide and many oxide n-type photoanodes are susceptible to photodegradation in aqueous electrolytes.
Approaches have been used to control the photoinstability of the semiconductor-electrolyte interface using surface coating techniques. For example, to stabilize semiconductor surfaces from photodecomposition, noncorroding layers of metals or relatively stable semiconductor films have been deposited onto the electrode surface. It has been reported that continuous metal films which block solvent penetration can protect n-type GaP electrodes from photocorrosion. However, if the films are too thick for the photogenerated holes to penetrate without being scattered, they assume the Fermi energy of the metal. Then the system is equivalent to a metal electrolysis electrode in series with a metal-semiconductor Schottky barrier. In such a system, the processes at the metal-semiconductor junction control the photovoltage and not the electrolytic reactions. In general, a bias is required to drive the water oxidation. In other cases, the metal can form an ohmic contact that leads to the loss of the photoactivity of the semiconductor. In discontinuous metal coatings, the electrolyte contacts the semiconductor, a situation which can lead to photocorrosion. For example, discontinuous gold films do not seem to protect n-type GaP from photocorrosion.
Corrosion-resistant wide-bandgap oxide semiconductor (TiO.sub.2 and titanates mostly) coatings over narrow-bandgap n-type semiconductors such as GaAs, Si, CdS, GaP, and InP have been shown to impart protection from photodecomposition. One of two problems is currently associated with the use of optically transparent, wide-bandgap semiconducting oxide coatings: either a thick film blocks charge transmission, or a thin film still allows photocorrosion.
Wrighton et al. (1978) have shown that chemical bonding of electroactive polymers to the semiconductor surface affects the interfacial charge-transfer kinetics such that the less thermodynamically favored redox reaction in the electrolyte predominates over the thermodynamically favored semiconductor decomposition reaction. To date, emphasis has been placed on improving the catalytic properties of p-type electrodes, where photocorrosion by reductive processes is not a major problem. The overvoltage for the evolution of hydrogen from p-type electrode surfaces is normally quite large. It has been demonstrated, however, that the catalytic property of a p-type Si photocathode is enhanced for hydrogen evolution when a viologen derivative is chemically bonded to the electrode surface and Pt particles are dispersed within the polymer matrix: (R. N. Dominey, N. S. Lewis, J. A. Bruce, D. C. Bookbinder and M. S. Wrighton, J. Am. Chem. Soc., 104, 467 (1982)). The viologen mediates the transfer of the photogenerated electron to H.sup.+ by the platinum to form H.sub.2. A thin platinum coating directly on the p-type silicon surface also improves the catalytic performance of the electrode: (Y. Nakato, S. Tonomura, and H. Tsubomura, Ber. Bunsenges. Phys. Chem. 80, 1289 (1976)). Charge conduction is generally much higher in electrically conductive polymers than in typical electroactive polymers.
Accordingly, work on charge conductive polymers in the field of photoelectrochemistry has been directed toward stabilization of electrodes against photodegradation in electricity-generating cells. Charge conductive polymers are known to protect certain semiconductor surfaces from photodecomposition, by transmitting photogenerated holes in the semiconductor to oxidizable species in the electrolyte at a rate much higher than the thermodynamically-favored rate of decomposition of the electrode. For example, R. Noufi, A. J. Frank, A. J. Nozik, J. Am. Chem. Soc., 103, 1849 (1981) demonstrated that coating n-type silicon semiconductor photoelectrodes with a charge conductive polymer, such as polypyrrole, enhances stability against surface oxidation in electricity-generating cells. As also reported by R. Noufi, D. Tench, L. F. Warren, J. Electrochem. Soc., 127, 2310 (1980), n-type GaAs has also been coated with polypyrrole to reduce photodecomposition in electricity-generating cells, although the polymer exhibited poor adhesion in aqueous electrolyte.
Despite the promising use of polypyrrole on n-type silicon to suppress photodecomposition, heretofore, whether or not conductive polymers in general could be used in conjunction with catalysts was unknown. Moreover, it can be seen that the discovery of uses for various polymer coatings on photoelectrodes has been on a case by case basis because of the empirical nature of the effects on any particular semiconductor and/or the interaction with a given electrolyte environment.